1. Field of the Invention
The present invention relates to a bootstrap drive circuit, and a switching regulator comprising the same for turning ON/OFF a high-side switch element.
2. Description of the Related Art
In recent years, switching regulators, which have highly efficient power conversion characteristics, are often used for power supply circuits which supply predetermined direct-current voltages to various electronic apparatuses. There are various switching regulators which are used, depending on the required characteristics. For example, FIG. 5A illustrates a circuit configuration of a step-down switching regulator as a first conventional example.
As illustrated in FIG. 5A, the conventional step-down switching regulator has: an input direct-current power supply 51 which supplies an input voltage Vi; a high-side switch 52 composed of an N-channel type MOS FET (metal-oxide-semiconductor field-effect transistor) having a drain connected to the positive terminal of the input direct-current power supply 51 and a gate connected to a high-side drive circuit 60; a low-side switch 53 composed of an N-channel type MOS FET having a drain connected to the source of the high-side switch 52, a gate connected to a low-side drive circuit 61, and a source connected to the ground; an inductor 54 which has one end connected to the source of the high-side switch 52 and the other end connected to an output terminal; an output capacitor 55 which has one end connected to the output terminal and the other end connected to the ground; a bootstrap capacitor 62 which has both terminals connected to the high-side drive circuit 60, and applies a bias voltage to the high-side drive circuit 60; and a bootstrap power supply 56 which is connected to the positive terminal of the bootstrap capacitor 62 via a forwardly connected diode 63, and supplies a drive voltage Vb as a bias voltage to the low-side drive circuit 61.
With this configuration, in the conventional step-down switching regulator, the high-side switch 52 and the low-side switch 53 are alternately switched to generate a pulse voltage, which is in turn smoothed by the inductor 54 and the output capacitor 55 to be output as an output voltage Vo. Here, the output voltage Vo and the input voltage Vi have a relationship represented by Vo=D×Vi, where D represents a duty ratio which is a proportion of an ON period of the high-side switch 52 with respect to one switching cycle. The output voltage Vo is detected and fed back by a control circuit, and is stabilized by adjusting the ON period of the high-side switch 52, though not illustrated.
When the high-side switch 52 is composed of an N-channel type MOS FET as in the first conventional example, a voltage higher than the input direct-current power supply Vi is required as a power supply voltage for the high-side drive circuit 60, so that the above-described bootstrap drive circuit is used. As illustrated in FIG. 5A, the high-side drive circuit 60 amplifies and outputs a high-side drive signal DH which has been biased by a voltage charged in the bootstrap capacitor 62 and has been input, thereby turning ON/OFF the high-side switch 52. Also, the low-side drive circuit 61 amplifies and outputs a low-side drive signal DL which has been biased by the drive voltage Vb from the bootstrap power supply 56 and has been input, thereby turning ON/OFF the low-side switch 53.
FIG. 5B is a timing chart of the high-side drive signal DH and the low-side drive signal DL. As illustrated in FIG. 5B, in order to avoid the high-side switch 52 and the low-side switch 53 from being simultaneously in the ON state, a high-level voltage and a low-level voltage are alternately repeated while simultaneously providing an inactive period during which a low level is provided. When the high-side drive signal DH is at the low level and the low-side drive signal DL is at the high level, the high-side switch 52 is in the OFF state and the low-side switch 53 is in the ON state. In this case, the source of the high-side switch 52 has a potential of 0, so that the bootstrap capacitor 62 is charged via the diode 63 by the bootstrap power supply 56.
Thus, when the bootstrap drive circuit is used, the bootstrap capacitor 62 needs to be charged by causing the low-side switch 53 to go to the ON state, so as to stably supply a bias voltage to the high-side drive circuit 60 which turns ON/OFF the high-side switch 52. However, when an electronic apparatus to which the switching regulator supplies the output voltage Vo is in the standby state (i.e., a light-load state), the ON period of the high-side switch 52 may be reduced, and the ON state period of the low-side switch 53 may also be limited so as to prevent backflowing of a current from the output terminal (an operation mode called a discontinuous operation).
Particularly, a load in the standby state tends to be considerably light as compared to that in the operating state, so as to support recent trends to energy conservation. In such an extremely light load state, a period during which both the high-side switch 52 and the low-side switch 53 are in the OFF state occupies a most portion of the switching cycle. In addition, in order to reduce switching loss, the switching cycle may be increased, or the operation itself of the switching regulator is suspended for a predetermined period (intermittent operation). During the light load state, the extremely light load state, or the intermittent operation, the ON state period of the low-side switch 53 is short, or the frequency of the ON state is low, so that the bootstrap capacitor 62 is not sufficiently charged, leading to a problem that a bias voltage can no longer be stably supplied to the high-side drive circuit 60.
As a second conventional example which includes a drive circuit for solving such a problem, there is a circuit described in, for example, JP 2004-304527 A (see FIG. 6).
The circuit of FIG. 6 includes, in addition to the components of FIG. 5A: a charge pump drive circuit 64; a first switch element 65 composed of an N-channel type MOS FET and a second switch element 66 composed of a P-channel type MOS FET which are alternately driven ON/OFF by the charge pump drive circuit 64; a first diode 67 having an anode connected to the low-side drive circuit 61; a flying capacitor 68 having a positive terminal connected to the cathode of the diode 67 and a negative terminal connected to the common drain of the first switch element 65 and the second switch element 66; and a second diode 69 having an anode connected to the positive terminal of the flying capacitor 68 and a cathode connected to the positive terminal of the bootstrap capacitor 62.
The second switch element 66 is connected between the source of the high-side switch 52 and the drain of the first switch element 65. Therefore, when the first switch element 65 is in the ON state and the second switch element 66 is in the OFF state, the flying capacitor 68 is charged by the bootstrap power supply 56 via the first diode 67. On the other hand, when the first switch element 65 is in the OFF state and the second switch element 66 is in the ON state, the bootstrap capacitor 62 is charged by the flying capacitor 68 via the second diode 69.
With the above-described configuration, in the step-down switching regulator of the second conventional example, by the switching operations of the first switch element 65 and the second switch element 66, the bootstrap capacitor 62 is charged via the flying capacitor 68 by the bootstrap power supply 56. Therefore, the bootstrap capacitor 62 can stably supply a bias voltage to the high-side drive circuit 60, irrespective of the ON/OFF states of the high-side switch 52 and the low-side switch 53.
However, the drive circuit included in the switching regulator of the second conventional example needs to include the flying capacitor 68 in addition to the bootstrap capacitor 62. In general, a capacitor has a larger area than that of a MOS FET or the like included in a switch element, and therefore, by adding another capacitor to a drive circuit, the sizes of the drive circuit and the switching regulator are increased.